The present invention relates to an energizing circuit for a reluctance motor.
Variable reluctance motors of the type for which the energizing system of the invention is applicable demand repeated commutation between the different phases in a predetermined sequence. Commutation is intended to mean switching the voltage source from one phase winding to another, which has to be done in order to keep the driving torque in a desired direction of rotation during the rotation.
The usual way to control the commutation from one phase to another in variable reluctance motors, particularly switched type drive motors, is to use at least one rotor position sensor providing signals in dependence of the rotation position of the rotor in relation to the stator, hereinafter referred to as the rotor position. This means, however, that extra elements must be provided at the rotor, which in practice have been associated with some inconveniences and give rise to errors, particularly in severe environments.
Motors working with variable reluctance are well-known and belong to the group of brushless DC-motors. The type of variable reluctance motors for which the invention is intended has a stator having one or several excitation windings provided in one or several phases having separate energizing of the windings belonging to a respective phase. Both the stator and the rotor are normally provided with salient poles or teeth. The rotor has no winding. The stator and the rotor provide a magnetic circuit in order to generate a mechanical rotational torque, which is essentially proportional to the square of the ampere-turns of the energized winding and to the variation of the permeance to the rotor position. The movement of the rotor in relation to the stator causes a variation of the reluctance and consequently of the permeance in the magnetic circuit of the stator winding.
A rotational torque in the driving direction of the motor is provided only when the ampere-turns of the winding are maintained during a rotor position interval, in which the permeance is increasing with the rotor position change. Therefore, the intention is to keep every winding excited only during such an interval for the winding. From driving technical reasons it is suitable, but not necessary, to have only one stator phase excited at the time, i.e. without overlap between the phase excitations.
Commutation from one phase to another can be made such that every phase winding is connected during a rotor position interval, in which the permeance is increasing with the rotor position change. The feeding to each phase winding must in this case be disconnected or lowered during every rotor position interval, in which the permeance is decreasing with the rotor position change. As mentioned above the most common way is therefore to use extra rotor position sensors to sense the rotor position currently and to control the feeding of the voltage supply with a sensor controlled circuit.
However, the desire is to get rid of these rotor position sensors. Several attempts have been made to use the variation of current or voltage characteristics of the stator windings and associated circuits to give an indication on suitable times for connection and disconnection of the voltage supply.
A problem inherent with the control of switched variable reluctance motors of the driving type without any rotor position sensors is the desire that the driving must be provided both at low and high rotation speed. Most of the earlier systems having sensing of the motor characteristics to control the excitation voltages for a motor have primarily been adapted to step motors and operate well for relatively low rotation speed but operate worse or not at all when the rotation speed exceeds 50% of the maximum rotation speed of the motor.